1. Field of the Invention
The present invention relates to a camera module having an auto-focus apparatus.
2. Description of the Related Art
Recently, with the increase in consumer demand for application products for mobile telecommunication terminals, the marketability for application products is largely expanded. In order to meet the various requests of consumers, the technology is moving toward high-performance and multi-function day by day. In particular, research into incorporating auto focus (AF), optical zoom, shutter and anti-shaking functions of the digital camera into the mobile telecommunication terminal has been done. Among these functions, research into the auto-focus and optical zoom has been intensive as the pixel number of the image sensor (CCD & CMOS) increases.
Further, to drive the high-pixel camera module for the mobile telecommunication terminal, a stepping motor, a VCM or a piezoelectric ultrasonic motor is representatively used. In particular, the piezoelectric ultrasonic motor transforms simple vibrations such as the expansion and contraction that occur when electric power is applied to a piezoelectric ceramic, to linear or rotational movement by using friction between stator and rotor. Accordingly, the piezoelectric ultrasonic motor has merits such as higher energy density, rapid response speed, higher positioning accuracy and an off-power holding function compared to electro-magnetic drive motor. In addition, the piezoelectric ultrasonic motor makes no noise when being actuated and the effect thereon of electromagnetic waves is negligible.
More specifically, in the piezoelectric ultrasonic motor, the piezoelectric actuator is coupled to a rod as a power transmission apparatus to which a driven member is coupled. Herein, a coupling means is required for the driven member so as to allow the driving force from the rod to be transmitted thereto. It is also necessary to maintain a suitable level of preload between the rod and the driven member to transmit the driving force of the piezoelectric actuator from the rod to the driven member. Also, it is very important that the lens barrel does not twist or rotate when being driven to secure the tilting performance of a lens.
Hereinafter, an auto-focus function using the piezoelectric ultrasonic motor of the prior art will be described in detail by referring to FIG. 1.
FIG. 1 is a perspective view diagrammatically illustrating an auto-focus apparatus of a camera module incorporating a piezoelectric actuator according to the prior art. As shown in FIG. 1, the piezoelectric actuator includes a piezoelectric element 10 and a driving shaft 20. And, the piezoelectric element 10 is expanded and contracted depending on the driving pulse applied and the one end of the driving shaft 20 is connected to the piezoelectric element 10, so that the driving force of the piezoelectric actuator can be transmitted to the lens barrel 40 as a driven member. The auto-focus apparatus includes a coupling means 30 for coupling the lens barrel 40 to the driving shaft 20. However, since the coupling means 30 includes a leaf spring 31 and a frictional member 32, the structure of the coupling means becomes very complex and thus the productivity decreases. Also, since the contact and support area between the coupling means and the driving shaft fixed to the coupling means is small, there is a problem of the lens barrel rotating or twisting when being driven.
Finally, in a camera module of the mobile telecommunication terminal according to the prior art, there is a problem because the auto-focus apparatus incorporates an elastic component (for example, spring) for securing and maintaining a preload between the driving member and the driven member, which makes the structure of the auto-focus apparatus complex and thus decreases productivity. In addition to this problem, another problem wherein the auto-focus apparatus cannot precisely control the position of a lens and the lens barrel rotates or twists when being driven, can take place.